I find it all depends on how you drive. I can get mine down to 10-12 if I hot rod everywhere I go. I can also get almost 20 if I drive more conservative or spend more time on the highway.
my 2008 5.3 got 17 if I took it easy. My 2015 6.2 gets 25 plus if I baby it and 18 or less if I hot rod it. Bottom line, the 6.2 gets WAY better gas mileage than the 5.3 and is way more fun to drive (hot rod)